The present invention is generally directed to a method for treating a doped amorphous silicon surface to enhance electrical contact. The method is applicable to the production of microelectronic circuit devices, and more particularly, is more applicable to the production of thin film amorphous silicon semiconductors, particularly those employed in liquid crystal display matrix addressed systems.
A liquid crystal display device typically comprises a pair of flat panels sealed at their outer edges and containing a quantity of liquid crystal material. The flat panels generally possess transparent electrode material disposed on the inner surfaces in predetermined patterns. One panel is often covered completely by a single transparent ground plane electrode. The opposite panel is configured with an array of transparent electrodes, referred to herein as pixel (picture element) electrodes. Thus a typical cell in a liquid crystal display includes liquid crystal material disposed between a pixel electrode and a ground electrode forming, in effect, a capacitor-like structure disposed between transparent front and back panels. In general, however, transparency is required for only one of the two panels and the electrodes disposed thereon.
In operation, the orientation of liquid crystal material is effected by voltages applied across the electrodes on either side of the liquid crystal material. Typically, voltage applied at the pixel electrode effects a change in the optical properties of the liquid crystal material. This optical change causes the display of information on the display screen. In conventional digital watch displays and in new LCD displays, screens used in some miniature television receivers, the visual effect is typically produced by variations in reflected light. However, the utilization of transparent front and back panels and transparent electrodes also the permits the visual effects to be produced by transmissive effects. These transmissive effects may be facilitated by subsequently powered light sources for the display including fluorescent type devices. This is typically referred to as back lighting. Various electrical mechanisms are employed to sequentially turn on and off individual pixel elements in an LCD display. Most relevantly, the switch element of the present invention comprises a thin film field effect transistor employing a layer of amorphous silicon. These devices are preferred in many LCD devices because of their potentially small size, low power consumption, switching speed, ease of fabrication, and compatibility with conventional LCD structures.
Thin film field effect transistors made from plasma enhanced chemically vapor deposited (PECVD) amorphous silicon (a-Si) and silicon nitride are ideal for matrix addressing of liquid crystal displays. They are fabricated on glass substrates with high picture element density using methods and equipment employed in conventional integrated circuit fabrication. In one process for FET fabrication and LCD displays, a molybdenum contact is made to N.sup.+ amorphous silicon using two masking steps. After a deposition of an insulative material such as silicon nitride, a layer of intrinsic amorphous silicon and the doping of the upper portions of the amorphous silicon layer, a thin layer of molybdenum is sputter deposited. This film is patterned back into small regions called mesas. Then the silicon nitride/silicon layers are patterned into regions somewhat larger than the mesas and referred to herein as islands. Subsequently, thick molybdenum is deposited on the wafer and patterned into source/drain and data line electrodes. The deposition of the thin molybdenum before subsequent processing into islands has been found to be necessary to ensure reliable contact of molybdenum to the N.sup.+ silicon. Hence, it is seen that two masking steps are required to form the contact: the mesa and mask and the island mask. Reducing the number of masking steps is desirable because it reduces processing time and in general, increases device yield.